The term “THz radiation” is used to mean electromagnetic radiation at a frequency lying approximately in the range 100 gigahertz (GHz) to 10 terahertz (THz), where 1 THz=1012 hertz (Hz), which corresponds to wavelengths lying in the range approximately 3 millimeters (mm) to 30 micrometers (μm). This spectrum range that is situated between the optical range and the microwave range has been under-used for a long time, in particular because of the lack of suitable sources and detectors.
Use of THz radiation is nowadays being actively developed both industrially and commercially. The main application lies in THz spectroscopy. Thus, the first commercial broadband THz spectroscopy systems have already appeared on the market. Amongst the pioneering companies in this field, mention may be made of Picometrix (USA), Teraview (UK) in co-operation with Brucker Optics, Nikon (Japan), Kwele (France), Gigaoptics (Germany), Tray-science (Canada), and Eskpla (Lithuania). Some of those firms also sell antennas for generating and detecting THz pluses: Eskpla, Gigaoptics, or indeed Tray-science.
All of those spectroscopy systems make use of a so-called “pump-probe” technique that relies on a femtosecond (fs) pulse laser that makes it possible to perform measurement over a broad spectrum band, but at low resolution (>1 GHz). That technique is capable of generating and detecting electromagnetic pulses having a duration of picosecond (ps) order and thus including spectrum components extending up to several THz.
Another technique of generating THz radiation that is highly promising for spectroscopic applications is photomixing, i.e. mixing two infrared lasers on an ultrafast photodetector.
The two superposed laser beams at frequencies f1 and f2 generate peaks at the frequency f=|f1−f2|, that is selected to lie in the THz range (100 GHz≦f≦10 THz). The photocurrent generated by the photodetector has a constant term and an oscillating term of frequency f: I=I0(1+cos(2πft)). The photodetector is coupled to an antenna that transmits electromagnetic radiation of frequency f. The assembly constituted by the photodetector and the antenna is referred to as a “photomixer”.
That technique was described for the first time in the article by E. R. Brown, K. A. McIntosh, K. B. Nichols, and C. L. Dennis entitled “Photomixing up to 3.8 THz in low-temperature-grown GaAs”, Appl. Phys. Lett. 66, 285 (1995). It has the advantage of presenting better spectral resolution (<1 GHz) and of being potentially portable since it can be used with semiconductor laser diodes in the range 700 nanometers (nm) to 1600 nm (0.7 μm-1.6 μm) depending on the photodetector used.
The photodetectors normally used for generating THz radiation by photomixing are mainly:                at λ=0.7 μm-0.8 μm: photoresistances using a material having a short lifetime such as gallium arsenide grown epitaxially at low temperature (LT-GaAs);        at λ=1.55 μm-1.6 μm: uni-traveling carrier photodiodes (UTC photodiodes) or photoresitances using a material having a short lifetime such as irradiated InGaAs; and        at λ=0.8 μm-1.06 μm, wavelengths at which ytterbium-doped optical fiber amplifiers operate, LT-GaSbxAs1-x photoresistances (with 0<x<1, the value of x serving to modify the absorption band of the material).        
The generation performance that is obtained is generally of the order of 1 microwatt (μW) at 1 THz for an optical power of the order of 50 milliwatts (mW) to 100 mW, whether at 0.8 μm or at 1.55 μm. The best results at 1 THz (2.6 μW at 1.05 TRz in 2003) have been obtained at 1.55 μm with a UTC diode coupled to a log-periodic antenna: see the article by H. Ito, F. Nakajima, T. Furuta, K. Yoshino, Y. Hirota, T. Ishibashi entitled “Photonic terahertz-wave generation using antenna-integrated uni-traveling carrier photodiode”, Electronics Letters, Vol. 39, No. 25, pp. 1828-1829, Dec. 11, 2003.
LT-GaAs photodetectors generally possess a planar structure constituted by interdigitated metal electrodes deposited on an epitaxial layer of LT-GaAs (see FIG. 1). A typical photodetector is made up of five metal electrodes having a width of 0.4 μm and a length of 10 μm, which electrodes are separated by 1.8 μm. The total area of the photodetector is 88 square micrometers (μm2).
The powers emitted by the photomixers, of the order of 1 μW at 1 THz, are still much too small for spectroscopy type applications. The only broadband detectors that are sufficiently sensitive to detect this level of power with a good signal-to-noise ratio (>1000) are bolometric detectors operating at 4 kelvins (K). An increase in the power generated would make it possible to use detectors at ambient temperature.
The THz power emitted by existing photodetectors is limited by thermal effects, such as the effects described by S. Verghese, K. A. McIntosh, and E. R. Brown in the article entitled “Optical and terahertz power limits in low-temperature-grown GaAs photomixers”, Appl. Phys. Lett. 71, 2743 (1997). For a given bias voltage, when optical power is increased progressively, photodetector destruction occurs before saturation phenomena appear, whether in terms of direct current (DC) or of THz power.
It would appear to be obvious to increase the surface area of the photodetectors in order to overcome those power limits. Nevertheless, if the transverse dimensions of the photodetector are not small compared with the wavelength of THz radiation, the contributions of the various surface elements of the detector can interfere destructively. To avoid that effect, recourse is had to so-called “distributed” planar structures, in which the photodetector is integrated in a THz waveguide. See for example:                E. A. Michael, B. Vowinkel, R. Schieder, M. Mikulics, M. Marso, and P. Kordos “Large-area traveling-wave photonic mixers for increased continuous Terahertz power”, Appl. Phys. Lett. 86, 111120 (2005); and        M. Mikulics, E. A. Michael, R. Schieder, J. Stutzki, R. Gusten, M. Marso, A. van der Hart, H. P. Bochem, H. Luth, and P. Kordos, “Traveling-wave photomixer with recessed interdigitated contacts on low-temperature-grown GaAs” Appl. Phys. Lett. 88, 041118 (2006).        
Unfortunately, the complexity of such structures does not enable their full potential to be used. The maximum power generated at 1 THz is of the order of 1 μW, i.e. substantially the same as for a “point” photodetector, but with much smaller efficiency.